The problem of the world's supply of fresh water (less than 500 ppm of dissolved salts) has become acute in recent years. Apart from collecting fresh water (which ultimately arises from rainfall) attempts are being made today to process water reclaimed from sewage and to remove dissolved salts from ocean water and brackish water (water having about 15% the dissolved salt content of ocean water. Of all secondary sources of fresh water, ocean water is the most abundant, and brackish water, which is available in large quantities, is more economically processed than ocean water. Several methods for desalination are in use today: distillation (which is an energy intensive process); freezing (which suffers from entrapment of salt water between ice crystals); reverse osmosis (an energy intensive process requiring high operating pressures), and electrodialysis. This latter method essentially an electrically-driven dialytic separating process in which a three-compartment vessel is formed with two semipermeable membranes defining an inner and two outer compartments. Of the two semipermeable membranes, one is permeable only to cations and the other is permeable only to anions. A positive and a negative electrode are placed in the outer compartments. Initially, all three compartments are filled with salt water in which the salt is dissociated into cations and anions. When a potential difference is impressed across the electrodes, the cations migrate through cation permeable membranes and the anions migrate through anion permeable membranes under the impressed electric field. As a result, a depletion of salt is experienced in the central compartment with an attendant salt enrichment in the outer cells. In urban areas, the electric field is supplied from local electric power generation facilities. But in remote areas, in which electric power generation is not available, electrodialytic cells are preferably energized by photovoltaic devices. Desalination devices which incorporate both photovoltaic and electrodialytic functions in a single cell have been described by G. W. Murphy, inventor of the present invention, in Solar Energy, Vol. 21, pp. 403- 407 (1978). Such arrangements offer increased efficiency in operation and reduced construction and operation costs. In such arrangements, a photovoltaic member, usually in sheet form is immersed in the electrolyte solution to comprise the electrodes referred to above. Problems encountered in such photoelectrodialytic desalination arrangements include: chemical stability of the photovoltaic members in the electrolyte solution; toxicity levels of the electrolyte solutions and the risk of leakage into the product (fresh) water compartment; development of efficient semipermeable membranes; and overall conversion efficiency of the arrangment. Of great concern however, is the face that the semipermeable membranes are not impervious to toxic electrolyte ions, and that leakage of these ions into the product chamber has been observed to seriously degrade the quality of the product water and the extend of demineralization.
It is therefore an object of the present invention to provide a photoelectrodialytic cell having improved demineralization performance.
It is another object of the present invention to provide a photoelectrodialytic cell having improved protection of the product compartment from inleakage of toxic or otherwise harmful materials.
Additional objects, advantages and novel features of the invention will be set forth in part in the description which follows, and in part will become apparent to those skilled in the art upon examination of the following or may be learned by practice of the invention. The objects and advantages of the invention may be realized and attained by means of the instrumentalities and combinations particularly pointed out in the appended claims.